1. Field of the Invention
This invention relates to a photo-conductive layer for constituting a radiation imaging panel, which is appropriate for used in a radiation imaging apparatus, such as an X-ray imaging apparatus. This invention also relates to a radiation imaging panel comprising the photo-conductive layer.
2. Description of the Related Art
There have heretofore been proposed X-ray imaging panels designed for use in a medical X-ray image recording operation, such that a radiation dose delivered to an object during the medical X-ray image recording operation may be kept small, and such that the image quality of an image and its capability of serving as an effective tool in, particularly, the efficient and accurate diagnosis of an illness may be enhanced. With the proposed X-ray imaging panels, a photo-conductive layer sensitive to X-rays is employed as a photosensitive material. The photo-conductive layer is exposed to X-rays carrying X-ray image information, and an electrostatic latent image is thereby formed on the photo-conductive layer. Thereafter, the electrostatic latent image, which has been formed on the photo-conductive layer, is read out by use of light or a plurality of electrodes. The techniques utilizing the X-ray imaging panels have advantages over the known photo-fluorography utilizing TV image pickup tubes in that an image is capable of being obtained with a high resolution.
Specifically, when X-rays are irradiated to a charge forming layer located in the X-ray imaging panel, electric charges corresponding to X-ray energy are formed in the charge forming layer. The thus formed electric charges are read out as an electric signal. The photo-conductive layer described above acts as the charge forming layer. As the material for the photo-conductive layer, amorphous selenium (a-Se), PbI2, HgI2, Cd(Zn)Te, or the like, has heretofore been used. (The materials for the photo-conductive layer are described in, for example, U.S. Pat. No. 6,268,614 and Japanese Unexamined Patent Publication No. 11(1999)-211832.)
However, of the radio-conductive materials described in U.S. Pat. No. 6,268,614 and Japanese Unexamined Patent Publication No. 11(1999)-211832, amorphous selenium has the problems in that it is necessary for the layer thickness to be set large because of a low radiation absorption efficiency, application of a high electric field is required, and therefore the reliability is not capable of being kept high. Also, each of PbI2, HgI2, Cd(Zn)Te has the problems in that a dark current is high, and the signal-to-noise ratio is not capable of being kept high.
Therefore, the use of BiI3 as the radio-conductive material has been proposed in, for example, U.S. Pat. No. 2,717,173. BiI3 has the advantages in that the load to the environment is low. However, BiI3 has the problems in that, in cases where a layer of BiI3 is formed with a coating technique, the effect of collecting the formed electric charges is small, electric noise becomes high, and therefore the image graininess characteristics become bad. Also, as the material for the radio-conductive material, ZnO is described in, for example, Japanese Unexamined Patent Publication No. 11(1999)-211832. ZnO has the advantages in that the load to the environment is low. However, ZnO has the problems in that, since the radiation absorption efficiency is low, a high sensitivity is not capable of being obtained.
As materials for scintillators, Bi4Si3O12 and Bi4Ge3O12 have been known. (The use of Bi4Si3O12 and Bi4Ge3O12 as the materials for scintillators is described in, for example, “Phosphor Handbook” edited by Shigeo Shionoya and William M. Yen, CRC Press, 1998 pp. 540-545.) Also, as a ferroelectric material, Bi4Ti3O12 has been known. (Bi4Ti3O12 is described in, for example, “Physical Review,” Vol. 122, No. 3, pp. 804-807, 1961.)
However, whether Bi4M3O12, such as Bi4Si3O12, Bi4Ge3O12, or Bi4Ti3O12 described in “Phosphor Handbook” edited by Shigeo Shionoya and William M. Yen, CRC Press, 1998 pp. 540-545, or “Physical Review,” Vol. 122, No. 3, pp. 804-807, 1961, exhibits or does not exhibit the photo-conductivity has not heretofore been known at all. Also, Bi4M3O12 described above has not yet been selected as the photo-conductive material. The applicant conducted extensive research and found that Bi4M3O12 has the photo-conductivity. The present invention is based upon the findings described above.